(1) Field of the Invention
The present invention pertains to a bulk material transfer chute that transfers a flow of material from a discharge conveyor to a separate receiving conveyor. In particular, the present invention pertains to a precision transfer chute that receives a flow of material from a discharge conveyor and transfers that material to a receiving conveyor and deposits the flow of material onto the surface of the receiving conveyor in a precise manner that avoids spillage of the material, avoids excess dust generation from the transfer of the material, reduces material degradation, reduces stress and wear of the receiving conveyor components thereby reducing maintenance and repair costs, and reduces the power requirements of the receiving conveyor.
(2) Description of the Related Art
The transporting of bulk material, for example coal, from one area to another often involves the transfer of a stream or flow of the material from one conveyor apparatus to another conveyor apparatus. In the transfer of the material from the one conveyor to the other conveyor, it is often necessary that the material be discharged from a discharge end of the one conveyor and transferred onto a receiving end of the other conveyor. To facilitate this transfer of the bulk material, large hoppers or transfer chutes have been designed that receive the flow of material from the discharge conveyor and deposit or discharge the flow of material onto the receiving conveyor.
The design of bulk material transfer chutes has remained basically unchanged for over the past 50 years. The typical transfer chute has a general box-like trapezoidal configuration with interior corners and edges where fine coal and dust can accumulate and create a fire or explosion hazard. The discharge conveyor is positioned at the top of the chute and the receiving conveyor is positioned at the bottom of the chute. The top opening of the transfer chute has a general rectangular configuration with interior corners and edges and at least one flat end wall positioned opposite the discharge end of the discharge conveyor. Material, for example coal, discharged from the discharge conveyor often impacts against the flat end wall before falling downwardly into the interior of the chute due to gravitational forces.
The coal falls downwardly through a transition section of the chute. The chute transition section has flat sidewalls that meet at angled interior corners and converge as they extend downwardly, with the cross sectional area of the chute's transition section reducing as the chute extends downwardly.
A loading section is positioned below the chute transition section. The loading section also has flat side walls with angled interior corners and sliding interior surfaces that direct the coal in the direction and speed of the receiving conveyor.
A loading skirt is positioned at the bottom of the loading section. The loading skirt has sidewalls that extend along a portion of the receiving conveyor length, and a top wall or cover that extends over the skirted portion of the receiving conveyor. The chute loading section discharges the bulk material onto the portion of the receiving conveyor inside the loading skirt. The loading skirt sidewalls prevent spillage of coal from the sides of the conveyor resulting from the turbulence of the material transferred onto the conveyor, and the top wall forms a dust containment chamber with the sidewalls to minimize dust created by the turbulence. The turbulence is created in the material by the uncontrolled flow of the material through the chute and the change in the material velocity when the faster moving material impacts with the slower moving receiving conveyor. The skirt functions to minimize dust and spillage of the bulk material that pours from the bottom of the loading section onto the receiving conveyor. The skirt is also intended to minimize the dust generated by material such as coal, pouring through the bottom of the loading section and impacting with the belt surface of the receiving conveyor.
Rubber seals are commonly arranged along the outer sides of the skirt sidewalls adjacent to the receiving conveyor. The rubber seals are mounted to the skirt sidewalls by means of clamping-type apparatus. The apparatus hold the rubber seals in contact with the receiving conveyor and form a seal with the receiving conveyor that prevents the passage of dust from the receiving conveyor. The rubber seals are designed as consumable parts, and through their constant contact with the receiving conveyor in providing an efficient seal, require regular maintenance and frequent replacement. Additionally, the constant pressure of the contact of the seals against the receiving conveyor on both sides of the conveyor system creates a frictional drag on the receiving conveyor that requires increasing the horsepower of the receiving conveyor motive source, thus increasing the cost of operating the conveyor.
The conventional bulk material transfer chute described above is disadvantaged in several respects. The bulk material discharged from the discharge conveyor that impacts with the transfer chute at the top of the chute interior creates dust, reduces the size of the material deposited into the chute, and causes wear to the wall of the chute that is impacted by the material. The impact of the material with the interior wall surfaces and corners causes a continuous build-up of material and can cause plugging of the chute. The plugging stops the flow of material through the chute and increases safety risks due to the potential for fire or an explosion, and increases maintenance costs to clear the plug. The material that falls through the chute transition section can spread out and entrain air that carries dust through the chute and out of the chute. In some chute transition and loading section designs, the freefall of the bulk material through the chute and onto the surface of the receiving conveyor can cause wear to the conveyor and can generate dust or spillage. The random flow of the material through the chute can cause off center loading of the material on the surface of the receiving conveyor. This often results in spillage of the material from the sides of the receiving conveyor which increases maintenance costs for maintaining the skirting, and presents a safety and health hazard due to dust generation which could be inhaled by persons or could create a fire or explosion hazard. The need for the loading skirt at the output of the chute transition section also adds to maintenance costs and increases the overall cost and health and safety risks of the transfer chute. The skirt drags on the conveyor causing wear to the conveyor and skirt, and increases power requirements of the conveyor. The skirt drag also requires that the skirt and conveyor be repaired or components replaced more frequently.
Recent advancements have been made in controlling the material stream and velocity through the chute by means of computer-generated discrete element modeling (DEM). Discrete element modeling accounts for the bulk material particle size and a theoretical coefficient of friction which simulates varying chute liner materials in an effort to predict the behavior of the material as it passes through the chute. The angular slope of the chute interior walls are arranged and adjusted so as to control the velocity of the bulk material passing through the chute and maintain a compressed material profile passing through the chute.
While DEM has proved to be successful in many applications, this method of chute design also has distinct disadvantages. DEM, as with conventional chute design, employs the use of a skirt system at the loading point on the receiving conveyor, with the same components and disadvantages as described previously.
Additionally, DEM is based on a single bulk material particle size and coefficient of friction controlling the material profile and velocity as it passes through the chute. Often these particular design criteria vary throughout the expected life of a bulk material handling system. In a power plant application, frequently material particle size changes due to altering suppliers of coal and often due to associated equipment performance such as coal crushers and granulators. This change in particle size would result in the necessity to remodel the design of the chute to accurately control the velocity and profile of material passing through the chute. Environmental conditions such as high moisture content due to heavy rain and freezing conditions, adversely affect the coefficient of friction between the bulk material particles and the boundary surfaces of the chute interior. This change in the coefficient of friction as a result of varying environment conditions renders the DEM inefficient in controlling material velocity.